Older dog has a lump on her eyeball that is oozing yellow/greenish stuff?

I have an older dog, 18 year old Blue Heeler, and she's had yellow/greenish stuff on and around one of her eyes for a few days. We've been cleaning it off 6/7times a day, but she wouldn't open her eye until today. When we clean it she snaps and bites at us as if it's hurting her, which she has never done before. Once she opened her eye we noticed a large white lump on her eyeball that the yellow/greenish stuff seems to be oozing from. The stuff oozing out seems kind of like pus. Does anyone have suggestions as to what it may be? I know I should probably take her to the vet, but she has other problems like, difficulty getting up, standing and walking, her back legs just don't support her anymore. I'm just afraid of the worst case scenario if I take her to the vet.